The gases in air, such as in particular oxygen and nitrogen, are very important industrially. At present, one of the non-cryogenic techniques used for producing these gases is the technique referred to as PSA (pressure swing adsorption), which encompasses not only PSA processes proper, but also similar processes, such as the VSA (vacuum swing adsorption) or MPSA (mixed pressure swing adsorption) processes.
According to this PSA technique, when the gas mixture to be separated is air and the component to be recovered is oxygen, the oxygen is separated from the gas mixture using preferential adsorption of at least nitrogen on a material which preferentially adsorbs at least nitrogen and is subjected to cycles of given pressure in the separation zone.
The oxygen, which is adsorbed little or not at all, is recovered at the outlet of the separation zone; it has a purity, in general, greater than 90%, or even 93%.
More generally, a PSA process for the non-cryogenic separation of a gas mixture comprising a first compound which is adsorbed preferentially on an adsorbent material, and a second compound which is adsorbed less preferentially on the adsorbent material than the first compound, with a view to producing the second compound, cyclically comprises:
a step of preferentially adsorbing at least the first compound on the adsorbent material, at an adsorption pressure referred to as the "high pressure", with recovery of at least some of the second compound produced in this way; PA1 a step of desorbing the first compound trapped in this way by the adsorbent, at a desorption pressure which is lower than the adsorption pressure and is referred to as the "low pressure"; PA1 a step of recompressing the separation zone comprising the adsorbent, by progressively changing from the low pressure to the high pressure. PA1 the Si/Al ratio is between 1 and 2.4, PA1 the Na/Li ratio is between 0.012 and 0.300, PA1 the Mg/Li ratio is between 0.012 and 0.400, PA1 the Ca/Li ratio is between 0.012 and 0.200, PA1 and the K/Li ratio is between 0.001 and 0.060. PA1 The binder comprises a clay of the group formed by attapulgite, bentonite, kaolin or mixtures thereof, or a similar clay. PA1 The Si/Al ratio is between 1.15 and 1.70. PA1 The adsorbent contains a zeolite of the faujasite, preferably X or LSX (low silica X), type. PA1 The Na/Li ratio is between 0.015 and 0.250, the Mg/Li ratio is between 0.037 and 0.327, the Ca/Li ratio is between 0.024 and 0.145 and/or the K/Li ratio is between 0.001 and 0.036. PA1 The Na/Li ratio is between 0.018 and 0.230, the Mg/Li ratio is between 0.050 and 0.267, the Ca/Li ratio is between 0.038 and 0.100 and/or the K/Li ratio is between 0.001 and 0.025. PA1 The adsorbent contains 50% to 85% of element Li, 1% to 25% of element Na, 1% to 20% of element Mg, 1% to 10% of element Ca and 0.1 to 3% of element K. PA1 The adsorbent contains 55% to 82% of element Li, 2% to 20% of element Na, 3% to 18% of element Mg, 2% to 8% of element Ca and 0.1 to 2% of element K. PA1 The adsorbent contains 60% to 81% of element Li, 2.5% to 15% of element Na, 4% to 16% of element Mg, 3% to 6% of element Ca and 0.1 to 1.5% of element K. PA1 The proportion by mass of binder is at most 30% of the total mass of an adsorbent particle, preferably at most 25%. PA1 The gas flow to be separated comprises nitrogen and at least one less polar compound, in particular oxygen and/or hydrogen and, preferably, the gas flow is air, the first gas compound being nitrogen and the second gas compound being oxygen. The air is, in the scope of the present invention, the air contained inside a building or a heated or unheated chamber, or the outside air, that is to say under atmospheric conditions, taken as such or optionally pre-treated. PA1 The first gas compound is nitrogen and the second gas compound is oxygen; and an oxygen-rich gas flow is produced, that is to say one generally comprising 90% of oxygen. PA1 It is of the VSA (vacuum swing adsorption), type. PA1 The high pressure for adsorption is between 10.sup.5 Pa and 10.sup.7 Pa, preferably of the order of 10.sup.5 Pa to 10.sup.6 Pa, and/or the low pressure for desorption is between 10.sup.4 Pa and 10.sup.6 Pa, preferably of the order of 10.sup.4 Pa to 10.sup.5 Pa. PA1 The feed temperature is between 10.degree. C. and 80.degree. C., preferably between 25.degree. C. and 60.degree. C. PA1 the Si/Al ratio is between 1 and 2.4, PA1 the Na/Li ratio is between 0.012 and 0.300, PA1 the Mg/Li ratio is between 0.012 and 0.400, PA1 the Ca/Li ratio is between 0.012 and 0.200, PA1 and the K/Li ratio is between 0.001 and 0.060.
However, it is known that the separation efficiency for a gas mixture, such as air, depends on a number of parameters, in particular the high pressure, the low pressure, the type of adsorbent material used and its affinity for the compounds to be separated, the composition of the gas mixture to be separated, the adsorption temperature of the mixture to be separated, the size of the adsorbent particles, the composition of these particles and the temperature gradient set up inside the adsorbent bed.
At present, although it has not been possible to determine any general behaviour law, knowing that it is very difficult to connect these various parameters with one another, it is also known that the nature and the properties of the adsorbent have an essential role in the overall efficiency of the process.
Currently, zeolites are the adsorbents most widely used in PSA processes.
The zeolite particles customarily contain mono-, di- and/or trivalent metal cations, for example cations of alkaline metals, alkaline-earth metals, transition metals and/or lanthanides, incorporated during the synthesis of the zeolite particles and/or inserted subsequently by an ion-exchange technique, that is to say, in general, by bringing the unexchanged zeolite particles or raw zeolite into contact with a solution of one or more metal salts comprising the cation or cations to be incorporated into the zeolite structure, and subsequently recovering the particles of exchanged zeolite, that is to say zeolite containing a given quantity of metal cations. The proportion of metal cations introduced into the zeolite structure, relative to the total exchange capacity, is referred to as the exchange factor, which is between 0 and 100%.
Moreover, the adsorbents most widely used in processes of the PSA type for separating gases, in particular air, are zeolites, in particular of the X or LSX type, highly exchanged, in general to more than 80%, or even to more than 95%, with cations of very expensive metals, such as in particular lithium cations. Such zeolites are, in particular, described in documents EP-A-486384, EP-A-606848, EP-A-589391, EP-A-589406, EP-A-548755, US-A-5,268,023, EP-A-109063 and EP-A-760248.
However, the performance of the process, in particular the adsorption capacity or selectivity, and the overall production cost of the gas can vary considerably depending on the adsorbent employed in the PSA process.